Applying ISO fits to radial bearings

Question

Do you need to take the bearing size tolerance into account when creating the tolerances for the mating hole/shaft when using ISO 286 tolerances?

For example, a 125 mm OD bearing with a desired H7 housing fit clearance should be +.04mm/-0. The bearing OD tolerance from the manufacturer is 125 mm +0/-.018 mm.

Should the hole size and tolerance be: 125 mm +.04/-0

or 125 mm +.022/-0 [.04 nominal clearance -.018 bearing tolerance]?

Answer 1

I think the answer to your question is yes, except it would not be an H7 but an H5 or H6.

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Here's the relevant page on roymech.org, which has brief explanation and tables.

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These fits are a combination of:

hole-tolerance-class / shaft-tolerance-class

For example H7/s6 ("medium drive press fit"), or H7/g6 ("sliding fit")

• The hole class is uppercase, the shaft is lowercase
• The number is how tight the tolerance band is, relative to a fundamental deviation
• The letter is where that fundamental deviation is, relative to the reference dimension e.g. [125mm]

The +0/-0.018 bearing OD tolerance, at 125mm, I am reading from the tables as "h5".

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For the specific numbers in the question, I think you have it right

max_ID - min_OD = max_clearance
min_ID - max_OD = min_clearance

max_ID = min_OD + max_clearance = (-.018) + (+0.040) = +0.022
min_ID = max_OD + min_clearance = (+0) + (-0) = -0

Which if I read the tables correctly, at 125mm, it is an H5 hole, thus an H5/h5 fit, although H6 is pretty close

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Now for bearings there is yet another consideration, which is the fit on the ID... Sometimes if you have a lot of interference on the inside, it can push the outside out, and vice versa. Perhaps not an issue for larger bearings. On the other hand, for many bearings, if neither the outside nor the inside has any interference, then the balls are a little bit loose, it leads to worse impact loads (if there are impact loads), causing the bearing to have a shorter life.